This invention relates to the melting furnace of a glassmaking operation, and in particular to a method and apparatus in which electric heating is employed to boost the output of a furnace whose primary source of heat is the burning of liquid or gaseous hydrocarbon fuels.
The conventional continuous glass melting furnace is provided with an inlet and an outlet at opposite ends, raw, pulverulent batch material being introduced through the inlet, and molten glass being drawn off at the outlet. The heat for melting and reacting the batch material is furnished by large jets of flame projected across and above the pool of molten glass in the furnace. Since the melting rate of such a furnace is restricted by the limited ability of the walls to withstand high flame temperatures, various proposals have been made to speed the melting rate and boost total output by providing auxiliary electric heaters beneath the surface of the pool of molten glass. Such heaters are generally comprised of two or more electrodes inserted into the molten glass, between which alternating current is passed to heat the glass by the Joule effect. Typical prior art electric booster heating arrangements are shown in the following U.S. Pat Nos.:
2,397,852. . . Gentil. . . Apr. 2, 1946
2,749,378. . . Penberthy. . . June 5, 1956
2,767,235. . . Herrold et al. . . Oct. 16, 1956
2,832,958. . . Penberthy. . . Apr. 29, 1958
Although such arrangements may supply some extra heat to the melting operation, they do not provide the most efficient utilization of electrical energy, and they concentrate the heating effect in portions of the molten glass that are closely adjacent to the walls of the furnace, thereby promoting erosion of the walls. This erosion is detrimental not only because furnace life is shortened, but also because it causes greater numbers of particles from the walls to enter the molten glass, which, because they are of a different composition and difficult to melt, appear in the final product as inhomogeneities or defects known as "stones." Each of the above-cited patents shows a relatively large number of short electrodes inserted through the furnace walls. Because current density will be greatest near the electrodes, all of these arrangements produce the hottest temperatures close to the walls, and thus promote erosion of the adjacent wall areas. The Gentil patent also requires the batch material to be melted by the electrodes in small doghouses before entering the furnace. That arrangement places partially melted batch material, which is even more corrosive than molten glass, into direct contact with the doghouse walls, and at the same time requires extremely high temperatures within the small space of the doghouses in order to effect complete melting there. Gentil's doghouse walls would therefore be subject to a high rate of erosion. The erosion could be slowed by cooling the wall areas around each electrode in the prior art arrangements, but to do so would waste a significant portion of the thermal energy provided by the booster heating.
Another problem encountered in continuous glass melting furnaces is the directional instability of the layer of unmelted or partially melted batch material, known as the batch blanket, which floats on the surface of the pool of molten glass. The end of the blanket farthest into the furnace often tends to drift against one of the sidewalls, which not only brings the corrosive batch material into contact with the sidewall, but also establishes a persistent, unsymmetrical heating and circulation pattern in the furnace which is highly undesirable.
It is an object of this invention to overcome the drawbacks associated with electric booster heating in a glassmaking process by providing an arrangement that efficiently directs electrically generated heat to the zone where it is best utilized, while at the same time avoiding increased furnace wall erosion and improving the directional stability of the batch blanket. These and other objects will become apparent from the following description of the invention.